Messi said the three points were the most important accomplishment in the Group F opener, not his second World Cup goal.

"It's important to win, but we have to improve," the Barcelona star said. "It's not easy because during the first World Cup match you are always anxious and nervous, but we played a great second half and we have to keep it up that way."

Messi himself acknowledged the changes from five in defence to a 4-3-3 formation made it much easier for him and fellow striker Sergio Aguero.

"I was often on my own and so was 'Kun' [Aguero] and it was very difficult, so there are things to be improved on," he said.

"We forwards prefer the system we used in the second half, with more attacking, because we could go out against them with more passing options, although in truth sometimes we suffer a little more," he said. "The first half costs us more because we were behind but what is important is that we won. There are a lot of things we must improve, but I am happy that we began with three points."

Messi said he was astounded by the Argentine fans at Maracana.

"The (fan support) in the stadium was crazy and beautiful," he said.

Argentina coach Alejandro Sabella only awarded his team a mark of six out 10 and said his half-time changes to provide more support to Messi had been crucial.

"On balance I would give it a six. We need to improve and part of that is in my hands -- it is up to me.

"The changes at half-time just produced this improvement. They were playing better together and there was more support around Messi. Once Messi received the ball there was better follow-up and better support.

"I think that, whatever happens in this World Cup, he is among the best players in the history of football."

Sabella confirmed that he had taken Aguero off as he had seemed tired rather than because of injury, even though the Manchester City frontman had appeared to be limping.